Well fluids are pumped from a well fluid producing earth formation through a tubing string which extends to the surface by a pump connected to the bottom end of the tubing string. The pump is actuated by a sucker rod string which is reciprocally moved in the tubing by a drive means located at the surface, the sucker rod string alternately being moved upwardly by the drive means and then allowed to move downwardly by its own weight. In some cases sinker bars may be connected to the lower portions of sucker rod string to minimize flexing of the sucker rod string due to the downward pressure of the weight of the upper portions of the sucker rod string on the lower portions. This flexing of some portions of the sucker rod string would cause the rods of the string to contact the internal surfaces of the tubing. Moreover, the tubing itself is not perfectly straight. In fact, it is somewhat helical or corkscrew in form so that the sucker rods, even if the sucker rod string itself were straight, would also contact the tubing at various locations along the length of the tubing. Such sliding contact of the rods with the tubing would of course cause damage both to the sucker rods and the tubing.
In order to prevent such contact of the sucker rods with the tubing, the rods are conventionally provided with guides or centralizers which are rigidly secured to the rods at several longitudinally spaced locations along each rod.
Some of such guides are of the type shown in the U.S. Pat. No. 3,442,558 to D. F. Sable, which are formed of a resilient substance and which resiliently grip the rod so that relatively great longitudinal forces must be exerted thereon to move the guide relative to the rod.
Other rod guides are molded on the rods and are formed of such substances as are disclosed in U.S. Pat. No. 4,088,185 to F. J. Carson.
Many such molded in place guides are formed of a plastic available commercially under the trademark Ryton.
The rigid molded in place rod guides require that a much greater force to be exerted thereon before they are displaced on the rod than the above described guides which resiliently grip the rod. The rigid rod guides, however, are subject to chipping.
Whether provided with rod guides of either of the above described types of rod guides, a large proportion of the guides are always in contact with the internal surfaces of the tubing. As a result, upon reversal of downward movement of the sucker rod string to upward movement, the force required to move the sucker rod string upwardly not only must overcome the weight of the sucker rod string and its inertia plus the force needed to force upward movement of the column of fluids in the tubing, but also the frictional resistance between the rod guides and the internal surfaces of the tubing string. Obviously the great upward forces exerted on the sucker rod string upon each such reversal of downward to upward movement of the rod string shorten the life of the sucker rods. Upon reversal of upward to downward movement of the sucker rod string, the forces exerted by upper portions of the rod string on the lower portions are greatly increased where the lower portions of the rod string have rod guides rigid therewith. To decrease such flexing forces in such cases requires the use of sinker bars of considerable weight. This, of course, increases the force which must be exerted on the sucker rod string to move it upwardly.
Since such rod guides which are rigidly mounted will wear and abrade at the locations of their sliding contact with the tubing, the guides will abrade or wear at such locations of their periphery. To provide even wear along the entire periphery of the rod guides, the sucker rod string must be rotated through some angle during each cycle of reciprocatory movement thereof.